1. Field of the Invention
The present invention relates to a data communication system. More particularly, it relates to an earth station for an artificial satellite data-communication system.
2. Description of the Related Art
A recent advance in satellite data communication technology allows direct satellite data communication between on-premise terminals and a hub station. In this system, a very small earth station having a small antenna with an aperture diameter of approximately 1.2 m is located at a subscriber's site, and the subscriber can then achieve direct data-communication with the hub station through an artificial satellite.
In this type of data communication system, the on-premise earth station should be as small as possible. In order to reduce the size of the earth station, outdoor equipment, including a transmitter and a receiver, may be mounted at the rear of the antenna and connected to an indoor terminal equipment through a coaxial cable, to form the earth station. Transmission and reception signals, both having an intermediate frequency (IF), are passed through the coaxial cable, together with a monitor signal for monitoring the outdoor equipment and a control signal for controlling the outdoor equipment.
The length of the coaxial cable depends on the distance between the indoor terminal equipment and the outdoor equipment. A long coaxial cable will cause a large attenuation of the level of the transmission IF signal, and accordingly, will reduce the power level of a transmission signal sent from the antenna of the earth station to the satellite. However, the power level of the transmission signal from the earth station, must be within a predetermined range, regardless of the attenuation in the coaxial cable, and thus a variety of methods of compensating the above attenuation are used.
The simplest method is to design the earth station by using a maximum length of coaxial cable, and to install that length of coaxial cable. If the distance between the indoor terminal equipment and the transmitter is shorter than this length, the surplus coaxial cable is coiled-up. This method, however, increases the costs of the coaxial cable installation and is difficult to install.
Another method is to correct the loss in the coaxial cable at each earth station, on the basis of the length of the coaxial cable, by using an amplifier. This method suffers from a drawback in that the measurement of the cable length must be precise and the correction value must be accurate. Normally, the correction amplifier is mounted on the rear of the antenna, together with the transmitter and is sealed by a waterproof covering. Any correction to the equipment necessitates opening the cover and a difficult adjustment of the amplifier.
In addition, in the above methods, a change in the loss due to a change of temperature occurs, and any change in the characteristics of the coaxial cable over a period of time cannot be corrected.
In an improvement of the above methods, an amplitude of the transmission IF signal is continuously detected and controlled to a constant value. This method is useful when applied to a continuous signal in, for example, a single channel per carrier (SCPC) communication system. However, in a time division multiple access (TDMA) communication system, the signal is sent as a periodical burst wave, and in a packet communication system, a signal is sent as an isolated burst wave. Accordingly, in those communication systems, the above simple level control cannot be adopted because they do not send a continuous signal.